1. Field of the Invention
The present invention pertains to apparatus for measuring the velocity of a projectile, and more particularly, to apparatus for measuring the muzzle velocity of a projectile fired from a gun.
2. Description of the Prior Art
Muzzle velocity is an important parameter in fire control systems wherein a field gun is aimed at a distant target. Typically, in a fire control system, muzzle velocity is assumed to be a constant parameter, and the more apparent variable parameters, such as wind data and target range, are entered into the fire control system and to compute gun-laying orders. However, muzzle velocity is not constant because the variations in powder charge and variations in projectile weight may cause considerable variations in the muzzle velocity for a given type of weapon. Also, barrel wear causes each individual gun to have different muzzle velocity characteristics over its useful life. Even over the short period of time, changes in the temperature of the barrel cause variations in the muzzle velocity.
To overcome the problem of a variation in muzzle velocity, it is desirable to measure muzzle velocity directly and enter the measured data into the fire control system to increase accuracy. Three types of velocity measurement systems are known: optical, radar, and strain gauge.
The optical-electronic type of muzzle velocimeter uses a pair of light beams perpendicularly to the path of the projectile so the projectile intercepts the beams. The time interval between interruptions of the beams, as sensed by a pair of photo-detectors, is used to compute velocity. However, optical sensing systems are subject to interference from powder blasts and are generally not rugged enough for field use.
Conventional radar apparatus can be used to measure muzzle velocity by aiming the radar at the projectile after it is fired and determining projectile velocity from analysis of signals reflected from the projectile. Radar systems are expensive and always expose the installation to some risk that the radar emission may reveal the gun location.
Another technique to measure muzzle velocity is the strain-gauge measurement of hoop stress. This unique approach is based on the observation that a projectile, when fired, causes a slight deformation in the barrel as the projectile travels therethrough and this deformation causes hoop stress. Two strain gauges spaced a distance apart on the gun barrel sense the hoop stress caused by a passing projectile. Muzzle velocity is then determined by dividing the distance between strain gauges by the time required for the projectile to pass between the strain gauges.